X-ray polarimetry is largely an unexplored area of an otherwise mature field of X-ray astronomy. Except for a few early attempts during the 1970s, no dedicated X-ray polarimeter has been flown during the past four decades. On the other hand, the scientific value of X-ray polarization measurement has been well known for a long time, and there has been significant technical progress in developing sensitive X-ray polarimeters in recent years. But there are no approved dedicated X-ray polarimetric experiments to be flown in the near future, so it is important to explore the polarimetric capabilities of other existing or planned instruments and examine whether they can provide significant astrophysical polarization measurements. In this paper, we present experimental results to show that the CZTI instrument onboard the forthcoming Indian astronomy mission, Astrosat, will be able to provide sensitive measurements of X-ray polarization in the energy range of 100−300 keV. CZTI will be able to constrain any intrinsic polarization greater than ∼40% for bright X-ray sources (>500 mCrab) within a short exposure of ∼100 ks with a 3-sigma confidence level. We show that this seemingly "modest" sensitivity can play a very significant role in addressing long pending questions, such as the contribution of relativistic jets to hard X-rays in black hole binaries and X-ray emission mechanism and geometry in X-ray pulsars.

AstroSat is a multi-wavelength satellite launched on 2015 September 28. The CZT Imager of AstroSat on its very first day of operation detected a long duration gamma-ray burst (GRB) namely GRB 151006A. Using the off-axis imaging and spectral response of the instrument, we demonstrate that CZT Imager can localise this GRB correct to about a few degrees and it can provide, in conjunction with Swift, spectral parameters similar to that obtained from Fermi /GBM. Hence CZT Imager would be a useful addition to the currently operating GRB instruments (Swift and Fermi ). Specifically, we argue that the CZT Imager will be most useful for the short hard GRBs by providing localisation for those detected by Fermi and spectral information for those detected only by Swift. We also provide preliminary results on a new exciting capability of this instrument: CZT Imager is able to identify Compton scattered events thereby providing polarisation information for bright GRBs. GRB 151006A, in spite of being relatively faint, shows hints of a polarisation signal at 100-300 keV (though at a low significance level). We point out that CZT Imager should provide significant time resolved polarisation measurements for GRBs that have fluence 3 times higher than that of GRB 151006A. We estimate that the number of such bright GRBs detectable by CZT Imager is 5 -6 per year. CZT Imager can also act as a good hard X-ray monitoring device for possible electromagnetic counterparts of Gravitational Wave events.

The Cadmium Zinc Telluride Imager (CZTI) is a high energy, wide-field imaging instrument on AstroSat. CZTI's namesake Cadmium Zinc Telluride detectors cover an energy range from 20 keV to > 200 keV, with 11% energy resolution at 60 keV. The coded aperture mask attains an angular resolution of 17 over a 4• .6 × 4• .6 (FWHM) field of view. CZTI functions as an open detector above 100 keV, continuously sensitive to GRBs and other transients in about 30% of the sky. The pixellated detectors are sensitive to polarisation above ∼ 100 keV, with exciting possibilities for polarisation studies of transients and bright persistent sources. In this paper, we provide details of the complete CZTI instrument, detectors, coded aperture mask, mechanical and electronic configuration, as well as data and products.

We present results based on the systematic analysis of high resolution 95 ks Chandra observations of the strong cool core cluster Abell 2390 at the redshift of z = 0.228 that hosts an energetic radio AGN. This analysis has enabled us to investigate five X-ray deficient cavities in the atmosphere of Abell 2390 within central 30′′ . Presence of these cavities have been confirmed through a variety of image processing techniques like, the surface brightness profiles, unsharp masked image, as well as 2D elliptical model subtracted residual map. Temperature profile as well as 2D temperature map revealed structures in the distribution of ICM, in the sense that ICM in the NW direction is cooler than that on the SE direction. Temperature jump in all directions is evident near 25 ′′ (90.5 kpc) corresponding to the average Mach number 1.44±0.05, while another jump from 7.47 keV to 9.10 keV at 68 ′′ (246 kpc) in the north-west direction, corresponding to Mach number 1.22±0.06 and these jumps are associated with the cold fronts. Tricolour map as well as hardness ratio map detects cool gas clumps in the central 30 kpc region of temperature 4.45 +0.16 −0.10 keV. The entropy profile derived from the X-ray analysis is found to fall systematically inward in a power-law fashion and exhibits a floor near 12.20±2.54 keV cm 2 in the central region. This flattening of the entropy profile in the core region confirms the intermittent heating at the centre by AGN. The diffuse radio emission map at 1.4 GHz using VLA L-band data exhibits highly asymmetric morphology with an edge in the north-west direction coinciding with the X-ray edge seen in the unsharp mask image. The mechanical power injected by the AGN in the form of X-ray cavities is found to be 5.94×1045 erg s −1 and is roughly an order of magnitude higher than the energy lost by the ICM in the form of X-ray emission, confirming that AGN feedback is capable enough to quench the cooling flow in this cluster.

We report a strong soft X-ray excess in the BL-Lacartae γ-ray blazar OJ 287 during the long exposure in May 2015, amongst two of the latest XMM-Newton observations performed in May 2015 and 2018. In case of May 2015 observation, a logparabola model fits the EPIC-pn data well while a logparabola plus powerlaw describes the overall simultaneous optical to X-ray spectra, suggesting the excess as the synchrotron tail. This interpretation, however, is inconsistent with the observed spectral break between near-infrared and optical spectra, attributed to standard disk around a supermassive black hole (SMBH). Based on this, we considered two commonly invoked accretion disk based models in AGNs to explain the soft excess: the cool Comptonization component in the accretion disk and the blurred reflection from the partially ionized accretion disk. We found that both cool Comptonization and blurred reflection models provide equally good fit to the data and favor a super-heavy SMBH of mass ∼ 10 10 M ⊙ . Further investigation of about a month long simultaneous X-ray and UV pointing observations revealed a delayed UV emission with respect to the 1.5-10 keV band, favoring X-ray reprocessing phenomenon as the dominant mechanism. The results suggest that the soft excess is probably caused by strong light bending close to the SMBH. The detected soft excess in 2015 data and its disappearance in 2018 data is also consistent with the presence of accretion disk emission, inferred from the NIR-optical spectral break between

We study X-ray and UV emission from the narrow-line Seyfert 1 galaxy II Zw 177 using a 137 ks long and another 13 ks short XMM-Newton observation performed in 2012 and 2001, respectively. Both observations show soft X-ray excess emission contributing 76.9 ± 4.9% in 2012 and 58.8 ± 10.2% in 2001 in the 0.3 − 2 keV band.We find that both blurred reflection from an ionized disc and Comptonized disc emission describe the observed soft excess well. Time-resolved spectroscopy on scales of ∼ 20 ks reveals strong correlation between the soft excess and the powerlaw components. The fractional variability amplitude F var derived from EPIC-pn lightcurves at different energy bands is nearly constant (F var ∼ 20%). This is in contrast to other AGNs where the lack of short term variation in soft X-ray excess emission has been attributed to intense light bending in the framework of the "lamppost" model. Thus, the variations in powerlaw emission are most likely intrinsic to corona rather than just due to the changes of height of compact corona. The variable UV emission (F var ∼ 1%) is uncorrelated to any of the X-ray components on short timescales suggesting that the UV emission is not dominated by the reprocessed emission. The gradual observed decline in the UV emission in 2012 may be related to the secular decline due to the changes in the accretion rate. In this case, the short term X-ray variability is not due to the changes in the seed photons but intrinsic to the hot corona.

We present results from NuSTAR and SWIFT/XRT joint spectral analysis of V4641 Sgr during a disk dominated or soft state as well as a powerlaw dominated or hard state. The soft state spectrum is well modeled by a relativistically blurred disk emission, a powerlaw, a broad Iron line, two narrow emission lines and two edges. The Markov Chain Monte Carlo simulation technique and the relativistic effects seen in the disk and broad Iron line allow us to self-consistently constrain the inner disk radius, disk inclination angle and distance to the source at 2.43 +0.39 −0.17 R g (GM/c 2 ), 69.5 +12.8 −4.2 degrees and 10.8 +1.6 −2.5 kpc respectively. For the hard state, the spectrum is a power-law with a weakly broad Iron line and an edge. The distance estimate gives a measure of the Eddington fraction, L 2.0−80.0keV /L Edd , to be ∼1.3 × 10 −2 and ∼1.9 × 10 −3 for the soft and hard states respectively. Unlike many other typical black hole systems which are always in a hard state at such low Eddington fraction, V4641 Sgr shows a soft, disk dominated state. The soft state spectrum shows narrow emission lines at ∼ 6.95 and ∼ 8.31 keV which can be identified as being due to emission from highly ionized Iron and Nickel in an X−ray irradiated wind respectively. If not due to instrumental effect or calibration error, this would be the first detection of a Ni fluorescent line in a black hole X−ray binary.

The broadband X-ray emission from type 1 active galactic nuclei, dominated by a powerlaw continuum, is thought to arise from repeated inverse Compton scattering of seed optical/UV photons by energetic electrons in a hot corona. The seed optical/UV photons are assumed to arise from an accretion disc but a direct observational evidence has remained elusive. Here we report the discovery of variations in the UV emission preceding the variations in the X-ray emission based on ∼ 100 ks XMM-Newton observations of the narrow-line Seyfert 1 galaxy Mrk 493. We find the UV emission to lead by ∼ 5 ks relative to the X-ray emission. The UV lead is consistent with the time taken by the UV photons to travel from the location of their origin in the accretion disc to the hot corona and the time required for repeated inverse Compton scattering converting the UV photons into X-ray photons. Our findings provide first direct observational evidence for the accretion disc to be responsible for the seed photons for thermal Comptonization in the hot corona, and constrain the size of the corona to be ∼ 20r g .